Photochemically produced bioavailable nitrogen from biologically recalcitrant dissolved organic matter stimulates production of a nitrogen-limited microbial food web in the Baltic Sea
Limnol. Oceanogr., 52(1), 2007, 132-143 | DOI: 10.4319/lo.2007.52.1.0132
ABSTRACT: Experiments were designed to assess whether the photochemical production of bioavailable nitrogen (N) from biologically recalcitrant dissolved organic nitrogen (DON) stimulates growth of N-limited plankton. Filtered Baltic Sea water was exposed to solar radiation for 12-19 days. Along with dark controls, samples were inoculated with an indigenous nanoplankton (<10 µm) community and incubated under photosynthetically active radiation and N-limiting conditions (molar dissolved inorganic nitrogen to PO43- ratio of 0.2). The concentration of particulate nitrogen, chlorophyll a (Chl a), and the biomass of phytoplankton and protozoa increased more in the solar radiation-exposed waters than in dark controls, showing that the photochemical transformation of dissolved organic matter (DOM) supported both heterotrophic and autotrophic plankton. In order to calculate the apparent quantum yield (AQY) for the photochemical reactions, the experimentally determined photoproduced bioavailable N (the formed particulate N and mineralized DON) and the phytoplankton biomass stimulated by DOM photochemistry were related to the number of photons absorbed by chromophoric DOM. Model calculations based on AQY indicated that DOM photochemistry produces 22- 26 µmol bioavailable N m-2 d-1 and stimulates phytoplankton biomass by 12-14 µg Chl a m-2 d-1 during summer in the northern Baltic Sea. Photoproduced bioavailable N potentially supports 1.2% and 3.6% of the new primary production and its N-demand, respectively, in this study region during summer. In N-limited surface waters, the photoproduction of bioavailable N from biologically recalcitrant, but photoreactive DOM can provide a new source of N and sustain the background productivity of a nanoplankton community, including phytoplankton.